LinearLayoutやRecyclerViewの、もともと用意されている区切り線について

LinearLayout

android:divider="?android:dividerHorizontal"
android:showDividers="beginning|middle|end"

RecyclerView

final DividerItemDecoration decoration = new DividerItemDecoration(getContext(), DividerItemDecoration.VERTICAL);
recyclerView.addItemDecoration(decoration);

もともと用意されているDrawable

android:background="?android:attr/listDivider"